High avidity CD8+ T cells are known to be the most effective mediators of viral clearance. Thus understanding how avidity is controlled following viral infection is of critical importance for designing optimally effective therapeutics and vaccines. Functional avidity is defined as the sensitivity of the cell to peptide/MHC (pMHC). Within the responding CD8+ T cell population are effector clones that encompass a broad range of avidities. Thus generation of a mixed avidity response appears to be the norm. The studies proposed in this application build on our previous findings using the paramyxovirus simian virus 5 (SV5). This model has been utilized to probe the anti-viral response following respiratory tract infection. We have made the seminal observation that following respiratory infection, the initial CD8+ effector cells generated exhibit a high avidity phenotype. As the response progresses, while high avidity cells continue to expand, low avidity also become apparent comprising approximately half of the anti-viral population. The overall goal of the project proposed here is to determine the mechanism responsible for kinetic separation in the appearance of high versus low avidity cells following viral infection. To this end, the studies proposed in specific aim 1 will determine the contribution of inherent and induced avidity to the generation of high versus low avidity T cells. The observation that high avidity cells are the initial responders while low avidity cells are restricted to later times suggests two hypotheses: 1) Avidity is induced in the responding cells by the conditions present at early versus late times postinfection and 2) Avidity is an inherent property of naove T cells and those of high versus low avidity are selectively activated at early vs. late times. This will be tested in part by determining the ability of cells present at early times to give rise to a mixed avidity population. In subsequent studies the inherent avidity of the naove pool will be manipulated to determine the effects on avidity at the population level at early versus late times. The goal of the studies in specific aim 2 is to determine the role of APC in the control of avidity. The kinetic separation in the presence of high versus low avidity cells would suggest differences exist in signals present in the lymph node at early versus late times. At later times, these signals would promote avidity down-modulation in high avidity cells generated earlier or alternatively would cause the activation of naove cells with inherently lower avidity. Aim two will test the hypothesis that the APC is a key component in this process. Together these studies will reveal new and important insights into the in vivo regulation of avidity and the role of APC in determining how T cells that differ in avidity are expanded during generation of the anti-viral response. Further they will provide novel information with regard to our understanding of the contribution of distinct APC subsets to the control of avidity following respiratory infection. Results from these studies may elucidate novel opportunities for intervention where immune responses are suboptimal as well as for the generation of more protective vaccines. PUBLIC HEALTH RELEVANCE High avidity CD8+ T cells are known to be the most effective mediators of viral clearance. The information gained from the novel studies proposed in this application will significantly impact the field by providing a model for how avidity is shaped in vivo following viral infection. This information is of critical importance for designing optimally effective therapeutics and vaccines.